目的:探讨Chiari畸形Ⅰ型(CMI)患者后颅窝MRI的特征性改变及其发病机制。方法:选取福建省立医院南院神经外科自2013年1月至2017年12月行后颅窝减压术治疗的110例CMI患者作为CMI组，选取同期门诊30例健康体检者作为对照组，应用图像存储与传输系统(PACS)测量所有受试者MRI T2加权像上后颅窝的骨性、神经结构。根据小脑扁桃体下疝的程度将CMⅠ患者分为Ⅰ度组、Ⅱ度组、Ⅲ度组，比较CMI组和对照组、不同小脑扁桃体下疝分度组患者临床资料和测量数据的差异。采用二分类Logistic回归分析CMI发病的独立影响因素。采用相关性检验分析CMI患者小脑扁桃体下疝程度、是否合并脊髓空洞与其他临床资料和测量数据的相关性。结果:(1)与对照组比较，CMI组患者的后颅窝斜坡长度、枕上长度、后颅窝高径长度均降低，斜坡倾斜角减少、颅脊角增大，差异均有统计学意义(n P<0.05)。二分类Logistic回归分析结果显示枕上长度、斜坡倾斜角是CMI发病的独立危险因素(n OR=0.057，n 95%CI：0.005~0.650，n P=0.021；n OR=0.730，n 95%CI：0.592~0.902，n P=0.004)。(2)与Ⅰ度组比较，Ⅱ度组、Ⅲ度组患者的枕骨大孔前后径长度增加，斜坡倾斜角减小，差异均有统计学意义(n P<0.05)。CMI患者的小脑扁桃体下疝程度与体质量指数(BMI)、斜坡倾斜角呈负相关关系(n r=-0.214，n P=0.045；n r=-0.271，n P=0.006)，与枕骨大孔前后径长度、颅脊角呈正相关关系(n r=0.336，n P=0.001；n r=0.282，n P=0.004)。CMI患者是否合并脊髓空洞与斜坡长度呈正相关关系(n r=0.235，n P=0.018)。n 结论:CMI患者后颅窝骨性发育异常导致容积狭小，不能适应正常发育的后脑神经组织，枕上长度的缩短和斜坡倾斜角变小是CMI的先天因素；颅脊角可能是小脑扁桃体下疝的后天因素，寰枕、寰枢关节退行性变可能参与颅脊角的改变。“,”Objective:To investigate the MR imaging characteristic changes of posterior cranial fossa in patients with Chiari malformation type I (CMI) and the pathogenesis of CMI.Methods:A total of 110 patients with CMI accepted posterior fossa decompression in our hospital from January 2013 to June 2017 were chosen as CMI group; 30 healthy subjects with matched gender and age were chosen as control group. On the median sagittal plane of MR imaging, Picture Archiving and Communication System (PACS) software was used to measure the length of clivus, anteroposterior diameter of the foramen magnum, supraocciput length, anteroposterior diameter of the posterior cranial fossa, height of the posterior cranial fossa, inclination angle of the clivus, and cranial spinal angle; and statistical analysis of rank sum test was performed on the above measured results. According to the degrees of cerebellum tonsillar herniation, 110 patients with CMI were classified into groups of degree I, II and III; the clinical data and measurement data of these patients were compared. The independent influencing factors for CMI were analyzed by binary Logistic regression. Correlations of cerebellar subtonsil hernia degree and whether combining with spinal cavity or not with other clinical data and measurement data in CMI patients were analyzed.Results:The length of clivus, supraocciput length, height diameter of the posterior fossa, and inclination angle of clivus were significantly decreased, and cranial spinal angle was significantly increased in CMI group as compared with those in the control group (n P<0.05); the results of binary Logistic regression analysis showed that supraocciput length and inclination angle of clivus were independent risk factors for CMI (n OR=0.057, n 95%CI: 0.005-0.650, n P=0.021; n OR=0.730, n 95%CI: 0.592-0.902, n P=0.004). (2) The anteroposterior diameter of the foramen magnum in patients from groups of degree II and III was significantly longer than that in patients from group of degree I (n P<0.05); the inclination angle of clivus in patients from group of degree III was significantly larger than that in patients from groups of degree I and II (P<0.05). In CMI patients, the degree of cerebellum tonsillar herniation was negatively correlated with body mass index and inclination angle of clivus (n r=-0.214, n P=0.045; n r=-0.271, n P=0.006), and positively correlated with anteroposterior diameter of the foramen magnum and cranial spinal angle (n r=0.336, n P=0.001; n r=0.282, n P=0.004). Whether combining with spinal cavity or not was positively correlated with length of the clivus in patients with CMI (n r=0.235, n P=0.018).n Conclusions:The abnormal bony development of the posterior fossa in malformed patients leads to small posterior fossa volume, which is not suitable for the normal development of posterior brain nerve tissue volume. The smaller length of clivus, supraocciput length and inclination angle of clivus are the congenital factors for occurrence of cerebellar tonsillar herniation. Increase of cranial spinal angle may be the acquired reason for worsening of cerebellar tonsillar herniation. The degeneration of atlantoaxial and occipital joints may be involved in the changes of cranial spinal angle.